Production of chlorates



United States Patent PRODUCTION OF CHLORATES Morris P. Grotheer and Edward H. Cook, Jr., Lewiston,

N.Y., assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed Nov. 30, 1965, Ser. No. 510,592 Int. Cl. C22d 1/02; C01b 11/26; B01k 3/10 U.S. Cl. 204-95 Claims ABSTRACT OF THE DISCLOSURE An alkali metal chlorate may be produced by the electrochemical decomposition of an alkali metal chloride in a diaphragm type chlor-alkali cell to produce chlorine and caustic which products are inter-reacted chemically at a pH of about 6 to 8 to produce hypochlorite and subsequently chlorate. A portion of the chlorate containing reaction mixture is made up with alkali metal chloride, acidified and fed to the anode compartment of the cell to afford an anolyte pH of about 1 to 5.

This invention relates to the production of chlorates and more particularly to a method of producing alkali metal chlorates by means of a chlor-alkali cell.

Chlorates, particularly sodium chlorate, have been produced by the electrolysis of aqueous solutions of alkali metal chlorides such as sodium chloride in electrolytic cells referred to as chlorate cells. Such cells produce chlorates in the cell by reacting chlorine produced at the anode with hydroxide ions produced at the cathode. The anode and cathode are not separated by a diaphragm and, therefore, the chlorine and caustic react with each other almost as soon as they are formed.

It is an object of the present invention to provide a method for the production of alkali metal chlorates in a more efiicient manner by means of a dual electrochemical and chemical reaction. It is another object of the present invention to provide a method for producing chlorates by controlled electrochemical decomposition of an alkali metal chloride to produce caustic and chlorine and subsequently to react the caustic and chlorine under controlled chemical reaction conditions. It is a further object of the present invention to provide a method for the electrochemical and chemical production of alkali metal chlorates at current efiiciencies of up to 98 percent and more. These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

In accordance with the invention a method is provided for the production of an alkali metal chlorate by the electrochemical decomposition of an alkali metal chloride in a chlor-alkali cell comprising subjecting an aqueous solution of an alkali metal chloride to a decomposition voltage to produce chlorine and an alkali metal hydroxide, removing said chlorine and alkali metal hydroxide from said cell, mixing and reacting said chlorine and alkali metal hydroxide at a pH of about 6 to 8 to form a reaction mixture containing hypochlorite and subsequently alkali metal chlorate, acidifying at least a portion of said reaction mixture and returning said acidified portion to the anolyte compartment of said cell, said acidification being in an amount to produce an anolyte pH of about The present invention advantageously utilizes both electrochemical and chemical processes for forming chlorates in a manner such that electrical power is not needlessly utilized to effect the chemical reaction of hypochlorite to alkali metal chlorate. Using the present method, current efficiencies are improved over conventional chlorate cells by up to 10 percent and more. Thus, alkali metal chlorates such as sodium chlorate, can be produced at a substantially lower cost.

The invention will be more readily described by reference to the drawing which is a flow sheet describing the invention.

The high efliciencies obtained by the present invention are realized by controlling the electrolyte temperature, pH and salt concentrations within the cell and by controlled reaction conditions for reacting chlorine and alkali metal hydroxide. Without such controls, especially pH conditions, the efficiency of the process greatly diminishes.

Although the present process can be used to effect the production of all alkali metal chlorates such as lithium chlorate, potassium chlorate, sodium chlorate, cesium chlorate, and the like, from their respective alkali metal chlorides, it is normally preferred to produce sodium chlorate from sodium chloride and subsequently convert the sodium chlorate to other desired chlorates. Therefore, the description herein will be directed more particularly to the production of sodium chlorate. However, it is to be understood that in describing the production of sodium chlorate, other alkali metal chlorates are also applicable and are to be included.

In the drawing, a chlor-alkali diaphragm cell 10 is operated under conditions favoring the present invention to thereby produce chlorine 12 and an alkali metal hydroxide 14 in an alkali metal chlorate-chloride solution. The chlorine 12 is evolved from the cell as a gas from the anolyte compartment where it is produced. Alkali metal hydroxide 14 is produced in the catholyte compartment of the chlor-alkali cell in admixture with unreacted alkali metal chloride and recycled alkali metal chlorate. The chlorine 12 and alkali metal hydroxide solution 14 are passed to reactor 16 wherein they are mixed and reacted.

Using pH control 18, a pH of 6 to 8, and more preferably a pH of about 6.5 to 7.5, is maintained in the re actor. At this pH, the chlorine reacts with the alkali metal hydroxide to produce alkali metal hypochlorite which rapidly converts to alkali metal chlorate. At the preferred pH, and at a temperature of about 60 degrees centigrade to about 110 degrees centigrade and more preferably about degrees centigrade to degrees centigrade, the hypochlorite content remains very 'low, even under continuous operating conditions. By the present pH control, the alkali metal hypochlorite content in reactor 16 is readily maintained below about 5 grams per liter. The pH is controlled within the desired range by addition of caustic and/or acid, such as HCl. The caustic can be supplied as cell liquor over and above that produced in the chlor-alkali cell and the acid can be supplied as chlorine, as well as HCl, over and above that produced in the cell. When HCl is used it can be obtained by burning hydrogen and chlorine gases evolved from a chlor-alkali cell. When caustic is added, it is added in an amount commensurate with the HCl or chlorine added to the anolyte feed solution and can be supplied as cell liquor.

The chlorine and caustic are completely mixed in the reactor using conventional techniques such as countercurrent flow of gas to caustic, using finely dispersed gaseous chlorine and/or rapid agitation. The reaction is rapid and exothermic in nature.

From reactor 16 the reaction mixture, which now contains an increased amount of alkali metal chlorate over that fed to the diaphragm cell 10, in addition to about 0.1 to 5 grams per liter of hypochlorite and alkali metal chloride, is passed to retention tank 17 for a holding period of about five minutes to two hours to reduce the hyprochlorite concentration to preferably below about 0.5 gram per liter. Retention tank 17 can be operated in a manner whereby the solution therein is recycled through reactor 16 thereby increasing the chlorate concentration in the liquor.

From retention tank 17, all or a portion of the liquor contained therein is passed to chlorate crystallization process 29. Alternatively, all or a portion of the liquor from retention tank 17 can be passed to saturator 24 for ultimate recycle to the diaphragm cell 10. The particular method used depends on how rapidly the grams per liter of NaClO- are being increased and what proportion of NaClO is being removed in the crystallization step. Normally, it is preferred to increase the amount of NaClO in retention tank 17 to above that contained in the cell feed liquor. Thus, the concentration of NaClO in retention tank 17 can be increased to about 500 to 750 or more grams per liter. Therefore, in passing through chlorate crystallization step 20, a portion or all of the chlorate can be removed with the mother liquor which contains sodium chloride and residual chlorate preferably being returned to the cell for further reaction. Several methods can be used to crystallize the chlorate from the reaction mixture. A lowering of the temperature of the solution will change the solubility of the chlorate without greatly affecting the solubility of the sodium chloride. Other methods are also known for removing the chlorate from the alkali metal chloride solution and such other methods can be used if desired.

In the present invention, it is preferred to increase the chlorate content of the reaction mixture to a point at which a crop of chlorate crystals can be removed from the reaction mixture with the least change in the reaction temperature. However, the chlorate content is kept at a level where at least 100 grams per liter and preferably 130 to 300 grams per liter of alkali metal chloride can be retained in the anolyte feed solution when the alkali metal chloride is sodium chloride. Slightly different amounts of other alkali metal chlorides can be used depending on their particular solubilities. Thus, the effect of chlorates being present in the feed solution can be overcome by controlling the cell operating conditions. Independent of the method used, it is preferred to remove a portion of alkali metal chlorate 22 continuously or periodically to maintain the desired chlorate and chloride concentration in the anolyte feed solution.

Recycle liquor from anolyte recirculation 32, retention tank 17 and crystallization process 20 is resaturated with additional amounts of NaCl 26 and/or water 28 as brine or solid salt in saturator 24 so as to provide a feed solution for the diaphragm cell containing at least 100 grams per liter of NaCl and up to about 750 grams per liter of sodium chlorate. Preferably, this feed solution contains 50 to about 600 grams per liter of sodium chlorate and 130 to 300 grams per liter of sodium chloride, Thus, satur-ator 24 replenishes the anolyte feed liquor with additional amounts of alkali metal chloride 26 and additional amounts of water 28 are as needed in the process to provide an anolyte feed solution containing at least 100 grams per liter of alkali metal chloride. The resaturated liquor is then acidified 30 preferably with chlorine gas or HCl prior to being returned to the anolyte compartment of the diaphragm cell. 10. The amount of acid or chlorine added is sufficient to produce a pH in the anolyte compartment of the chlor-alkali cell of about 1 to 5, and more preferably about 2 to 4.

In addition to operating the chlor-alkali diaphragm cell as described, the anolyte recirculation method can also be used wherein anolyte liquor 32 is withdrawn from the anolyte compartment of the chlor-alkali cell and replenished with alkali metal chloride, acidified, prefer-aby with HCl or chlorine, and returned to the anolyte compartment of the chlor-alkali cell 10. Using such a process, the feed to the chlor-alkali cell is at a rate greater than that at which cell liquor, alkali metal hydroxide 14, is withdrawn from the cell. This excess feed rate is about 1.5 to 10 times the rate of withdrawal from the catholyte compartment. The excess feed liquor is removed as anolyte liquor 32 for recirculation.

The chlor-alkali cell 10 used in the present invention can be any of the numerous types of chloro-alkali diaphragm cells wherein gaseous chlorine and aqueous alkali metal hydroxide cell liquor are produced. A typical chloralkali cell is described by Stuart in US. 1,862,244. Also, reference is made to the ACS Monograph Series, Book No. 154, entitled Chlorine, by Sconce (1962), Reinhold Publishing Co., pages 81 to 126, for further descriptions of chlor-alkali diaphragm cells. Such cells normally have graphite anodes and metal cathodes such as steel. .Asbestos or synthetic fibers are used as the diaphragm material. In the operation of the present process, normally a group of 50 to 100 or more cells are operated as described herein with the chlorine and caustic produced thereby being reacted as described herein.

Since the feed solution to the chlor-alkali cell of the present invention includes an alkali metal chlorate in addition to alkali metal chloride and particularly, in that the alkali metal chlorate is often in a concentration in excess of the alkali metal chloride, somewhat different operating conditions are preferred from those normally used in chlor-alkali cells. The cell operating temperature is preferably controlled in the range of about degrees centigrade to 100 degrees centrigrade, and more preferably in the range of degrees centrigrade to degrees centrigrade. These preferred ranges are somewhat lower than those preferred in normal chlor-alkali cell operation.

When using a cell with a steel or iron cathode, it has been found to be desirable to add a reagent to reduce or prevent chlorate reduction in the cell. Such reagents as alkali metal dichromates or chromic acid are particularly effective while also serving as a corrosion inhibitor. Although, while in operation there is no danger of attack on the steel cathode, it has been found that if the cathode should rust, as could happen during temporary shutdowns or during start-up periods, the iron oxides act as catalysts to reduce the alkali metal chlorate in the catholyte compartment in the presence of hydrogen to alkali metal chloride. Thus, hydrogen produced at the cathode rapidly reacts with the chlorate in solution. The amount of inhibitor used is that sufi'icient to inhibit the oxidation of the metal during temporary shutdown and start-upperiods. When sodium dichromate is used as the inhibitor, about 0.01 to about one percent by Weight of the feed solution is used. Alternatively, a stainless steel or chromium-containing steel can be used as the cathode material to thereby eliminate the desirability of using a reagent to prevent chlorate reduction.

The diaphragm used in the present cell can be any porous material resistant to the conditions within the cell. The most preferred materials are asbestos, Teflon, after chlorinated polyvinyl chloride, polyvinylidene chloride, and the like materials. The most preferred diaphragm materials are the more porous and acid resistant asbestos such as anthophyllite and asbestos mixtures containing anthophyllite. The diaphragm used in the cell is preferably more porous than that conventionally used for normal chlorine and caustic production. When deposited asbestos is used, the diaphragm weight is preferably 25 to 100 percent of that used for normal chlor-alkali production.

The invention will be more fully described by reference to the examples. Unless otherwise indicated, all temperatures are in degrees centigrade and all parts are by weight.

Example 1 The process of the present invention was operated in accordance with the drawing using a Hooker Type S-l chlor-alkali cell operated continuously at a current of 6,000 amperes. The chlor-alkali cell used had a 30 pound asbestos diaphragm deposited on a mild steel 6 strand per inch woven wire mesh cathode. The cell was operated by continuously feeding to the anolyte compartment of the cell an acidic solution of about 150 grams per liter of sodium chloride, 500 grams per liter of sodium chlorate, two grams per liter of sodium dichromate and about one-half gram per liter of sodium hydrochlorite at a rate of 11 to 12 liters per minute. The feed solution was acidified with HCl to a pH of about three. The flow from the anolyte compartment through the diaphragm into the catholyte compartment was about 2.5 liters per minute. The excess feed solution was withdrawn from the anolyte compartment at a rate of about 8% to 9 /2 liters per minute and passed to a resa-turator for replenishment of sodium chloride.

Caustic was produced in the catholyte compartment of the cell at a rate such that the cell liquor withdrawn from the catholyte compartment contained about 58 grams per liter of sodium hydroxide in addition to the sodium chlorate and residual amounts of sodium chloride. The cell liquor was passed to a reactor wherein it was mixed with chlorine evolved from the cell. The operating temperature of the cell was 80 degrees centigrade, thus providing a cell liquor which was fed to the reactor at about 80 degrees centigrade. An exothermic reaction ensued in the reactor thereby aiding in maintaining the reactor tempera ture at about 80 degrees centigrade. The pH was controlled in the reactor by the addition of small amounts of additional sodium hydroxide and/or acid to maintain the pH at about 7. The hydrochlorite concentration in the reactor achieved a level of about 3 grams per liter and remained about constant during prolonged continuous reaction conditions.

The solution in the reactor was passed to a retention tank which provided an average retention time of about one to one and one-half hours prior to being returned through the sodium chloride saturator to the cell for further electrolysis or being withdrawn for chlorate crystallization. The hydrochlorite concentration in the retention tank was in the range of about 0.5 to 2.0. grams per liter. The temperature in the retention tank was maintained at about 80 degrees centigrade to promote the reaction of hypochlorite to chlorate. A circulating stream was maintained between the retention tank and the reactor. A second stream of liquor was withdrawn from the retention tank and passed to the saturator for ultimate recycle to the cell.

A third stream of liquor is withdrawn from the retention tank for crystallization of sodium chlorate. Sodium chlorate is crystallized from the retention tank solution at a rate of about 8.5 pounds per hour by reducing the temperature of the solution to crystallize a crop of sodium chlorate crystals. The mother liquor from the chlorate crystallization process is passed to the saturator wherein it is mixed with the withdrawn anolyte solution, the retention tank effluent and replenishing amounts of sodium chloride and water to form the anolyte feed solution. This solution is acidified With HCl and returned to the anolyte compartment of the cell thereby providing a feed concentration of about 150 grams per liter of sodium chloride, 500 grams per liter of sodium chlorate, two grams per liter of sodium dichromate, about one-half gram per liter of sodium hypochlorite, at a pH of about 3.

As is readily seen from the example, the cell efliciency under the controlled cell pH conditions and chlorine-caustic reaction conditions and pH was higher than that attainable in conventional sodium chlorate cells.

Examples 2-5 The process of the present invention is again eifected using a chlor-alkali diaphragm cell having a graphite anode and a steel mesh cathode using a deposited anthophyllite asbestos diaphragm. The pH of the electrolyte in the anolyte compartment of the chlor-alkali cell was varied under controlled conditions to determine the effect on the anode current efficiency.

The process of operating the chlor-alkali cell to produce chlorates was effected in the same manner as Example 1 with the exception that a current density of one ampere per square inch was used at a cell temperature of 60 degrees centigrade. The anolyte feed solution was maintained fairly constant at about 500 grams per liter of sodium chlorate and 150 grams per liter of sodium chloride. Varying amounts of hydrochoric acid were added to the feed solution to provide the anolyte pH shown for each by Examples 2 through 5. The current efliciency was determined by Orsat gas analysis. Table I shows the results obtained.

TABLE I Example Current Number Electrolyte out of Cell (Anolyte) Efficieney It is readily seen from the examples, by maintaining a low pH in the anolyte compartment, extremely high current efliciencies are obtained even in the presence of high concentrations of sodium chlorate.

Examples 6-10 The method of the present invention was operated in accordance with the drawing and in the manner of Example 1. The process was operated using different cell temperatures and anolyte pHs to determine the optimum pH and temperatures at an anolyte feed concentration of about 150 grams per liter of sodium chloride, 500 grams per liter of sodium chlorate and about two grams per liter of sodium dichromate. The chlor-alkali cell used is a Hooker Type S4 cell having a graphite anode and a mild steel mesh cathode on which is deposited a 120 pound asbestos diaphragm. The cell is operated at 55,000 amperes using an anolyte feed rate of about 75 liters per minute.

The reaction of the sodium hydroxide produced in the catholyte compartment of the cell and the chlorine produced in the anode compartment was eliected at a temperature of about degrees centigrade and a pH of 6.8 to 7.2. Under these conditions of pH and temperature, substantially all of the chlorine is absorbed and reacted with the cell liquor using a co-current mixing action in the reactor. A hypochlorite level of about 2 grams per liter is obtained under continuous reaction conditions in the reactor. The reaction proceeded to completion in the formation of sodium chlorate.

Table 11 illustrates the total cell efliciency at various electrolyte pHs in the anode compartment of the cell and at various operating temperatures. The total cell efficiency is based on the current efficiency, the graphite consumption, the acid and caustic requirements for pH b adjustments and power cost. The optimum conditions produced a cost factor of 100.0 with the less than optimum cost conditions being indicated in relation to the optimum which for the given chlor-alkali cell is at a cell temperature of 80 degrees centigrade and an anolyte pH of 3.0.

TABLE II Optimum total cell Example Anolyte Cell temperature, operation cost factor Number pH degrees centigrade (100.0 equals optimum) As can be seen from the data, the best operating conditions were found to be at an anolyte pH of 3.0 to 4.0 and a cell operating temperature of 60 to 95 degrees centigrade.

Example 11 Using the information obtained in Examples 6 through 10, experiments were run using the chlor-alkali cell and process of Example 1 wherein the concentration of sodium chloride in the anolyte feed solution was varied from 50 grams per liter to 300 grams per liter and the sodium chlorate concentration was varied from about 50 grams per liter to about 750 grams per liter, the sodium chloride being near the saturation point at the given sodium chlorate concentrations. When the solution in the anolyte compartment had a pH of about 3 to 4, good cell efficiencies were obtained in the temperature range of 60 to 95 degrees centigrade at sodium chloride concentrations above grams per liter. The best current efficiencies were obtained at sodium chloride concentrations above grams per liter with the current efiiciencies dropping rapidly with sodium chloride concentrations below about 100 grams per liter. The concentration of sodium chlorate did not greatly affect the current efiiciencies provided acidic conditions were maintained and the sodium chloride concentration was maintained above 100 grams per liter.

While there have been described various embodiments of the present invention, it will be recognized by those skilled in the art that many variations are possible without departing from the spirit and scope of the invention as defined in the claims. It is intended to cover the invention broadly in whatever form its principles may be utilized.

What is claimed is:

1. A method for the production of an alkali metal chlorate by the electrochemical decomposition of an alkali metal chloride in a chlor-alkali diaphragm cell comprising subjecting an aqueous solution of an alkali metal chloride to a decomposition voltage to produce chlorine and an alkali metal hydroxide, removing said chlorine and alkali metal hydroxide from said cell, mixing and reacting said chlorine and alkali metal hydroxide at a pH of about 6 to 8 to form a reaction mixture containing hypochlorite and subsequently alkali metal chlorate, acidifying at least a portion of said reaction mixture and returning said portion to the anolyte compartment of said cell, said acidification being in an amount to produce an anolyte pH of about 1 to 5.

2. The method of claim 1 wherein diaphragm material in the chlor-alkali cell contains anthophyllite asbestos.

3. The method of claim 1 wherein the aqueous solution of alkali metal chloride subjected to a decomposition voltage in said chlor-alkali cell is an acidified mixture of sodium chloride and sodium chlorate.

4. The method of claim 3 wherein the aqueous solution feed to said cell contains 100 to 300 grams per liter of sodium chloride and 50 to 650 grams per liter of sodium chlorate.

5. The method of claim 1 wherein the alkali metal chloride and alkali metal chlorate are sodium chloride and sodium chlorate, respectively.

6. The method of claim 5 wherein said reaction mixture is acidified with a compound selected from the group consisting of chlorine, hydrochloric acid and mixtures thereof.

7. The method of claim 5 wherein a portion of sodium chlorate is crystallized and removed from the reaction mixture prior to returning to the anolyte compartment of the cell.

8. The method of claim 5 wherein the chlor-alkali cell is operated by feeding anolyte solution to said cell at a rate in excess of that flowing through the diaphragm with the excess brine being withdrawn from the anolyte compartment, resaturated with sodium chloride, acidified and returned to the anolyte compartment for further electrolysis.

9. The method of claim 5 wherein the anolyte pH is maintained at about 2 to 4 and the cell temperature is maintained between about 60 degrees centigrade to 100 degrees centigrade.

10. The method of claim 9 wherein the reaction between the chlorine and the sodium hydroxide is effected at a pH of 6.5 to 7.5.

References Cited UNITED STATES PATENTS 2,511,516 6/1950 Schumacher 204-95 2,628,935 2/1953 Earnest 204-95 2,954,333 9/1960 Heiskell et a1. 20498 3,043,757 7/ 1962 Holmes 204-95 3,055,821 9/1962 Holmes et a1 204270 3,219,563 11/1965 Collins et a1. 204-95 JOHN H. MACK, Primary Examiner D. R. JORDAN, Assistant Examiner U.S. Cl. X.R. 204l28, 266 

